• Support PF! Buy your school textbooks, materials and every day products Here!

Applications of Newton's laws

  • #1
4
0

Homework Statement


You've attached a bungee cord to a wagon and are using it to pull your little sister while you take her for a jaunt. The bungee's unstretched length is 1.3m and you happen to know that your little sister weighs 220N and the wagon weighs 75N. Crossing a street, you accelerate from rest to your normal walking speed of 1.5m/s in 2.0s, and you notice that while you are accelerating, the bungees length increases to about 2.0 cm. what is the force constant of the bungee cord, assuming it obeys hooke's law?

Homework Equations


f=ma
fs=kx

The Attempt at a Solution



Ok, so I am quite stumped with this question, i know you should find the total force of the little sister and wagon , 220+75, but i do not know if i should keep it as a force or as a total mass in kg. And the difference in spring length 2.0-1.3=.7m . I am just unaware if i bring kinematic forumlas into this and i am quite unaware of what to find because i am getting the wrong answer when looking for "k"

Any guidance is appreciated. Thanks
 

Answers and Replies

  • #2
34,056
9,922
The problem has a serious flaw, but we can ignore that for now.

I guess the 2.0 cm mean 2.0 m. The weight is a force downwards - as weight it won't help you much as your bungee cord is horizontal. Finding the mass is a good start.
What is the force needed to stretch the bungee cord? What does that force act on?
 
  • #3
4
0
yeah its 2.0 m . long day lol . ok , so it would be total mass over gravity, giving you; 295/9.80= 30.102kg ? acceleration is 1.5/2 = 0.75 m/s^2. I dont get what you are asking in the second part though.
 
  • #4
4
0
cant believe i had that brain fart. thanks !
 
  • #5
34,056
9,922
Let me rephrase that: Why does the wagon accelerate, and which force (in N) comes from where?

Edit: Okay, solved?
 
  • #6
4
0
from the pull? and i multiplied the mass and acceleration and divided by the .7m difference in the spring?
 
  • #7
34,056
9,922
Looks fine.
 

Related Threads on Applications of Newton's laws

  • Last Post
Replies
5
Views
1K
Replies
3
Views
2K
  • Last Post
Replies
5
Views
2K
Replies
3
Views
648
  • Last Post
Replies
21
Views
1K
  • Last Post
Replies
4
Views
1K
  • Last Post
Replies
13
Views
3K
  • Last Post
Replies
4
Views
3K
  • Last Post
Replies
12
Views
2K
  • Last Post
Replies
6
Views
2K
Top